The most efficent solar panels use GaAs. the same material in IR leds, so that is my choice. Wikipedia suggests that single colour light is more efficient than the whole spectrum and never said any colour specifically. the quantum dots technique is used in LEDs is like phosphor to create white and other LEDs. in solar panels it would serve to change the UVA, violet and blue to lower frequencies (this is an assumption). Glass does not pass UVB. A solar panel based on Pixel Qi's coloured backlight may be more efficient.